Hydrostatic transaxle

ABSTRACT

A hydrostatic transaxle axle assembly for a vehicle has the housing is formed by three housing members, two of which are preferably separable on a parting plane coincident with the longitudinal axes of the axle shafts. One of the housing members being provided with an opening to allow a portion of one of the other housing members to extend through, and where the extending housing member contains within its interior a number of internal fluid passages for fluidly connecting the hydraulic pump to the hydraulic motor.

RELATED APPLICATIONS

This application is a division of application Ser. No. 09/814,939 filedMar. 23, 2001, now U.S. Pat. No. 6,427,442 which is division ofapplication Ser. No. 09/411,835, filed on Oct. 4, 1999 now U.S. Pat. No.6,237,332, which is a division of U.S. application Ser. No. 09/112,363,filed on Jul. 9, 1998 which issued into U.S. Pat. No. 5,979,270, whichclaims the benefit under 35 U.S.C. §119(e) of Provisional ApplicationNo. 60/051,990, filed on Jul. 9, 1997, the entire contents of which arehereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to hydrostatic transaxles, and more particularly,to a compact, fully integrated hydrostatic transaxle incorporating ahydrostatic transmission of the type employing an axial pistonswash-plate pump and a hydraulic motor. Hydrostatic-transmissions inassociation with transaxle driving apparatus have proven to be veryuseful to-date and are used in numerous applications such as smallvehicles like self-propelled grass-mowing lawn tractors.

BACKGROUND OF THE INVENTION

Hydrostatically powered driven equipment such as lawn tractors havebecome extremely popular and many utilise the axial piston swash-plateconfiguration for both the pump and motor elements of the hydrostatictransmission. Such tractors generally have an internal combustion enginehaving a vertical crankshaft which is connected to the transaxle bymeans of a conventional belt and pulley arrangement. A standardhydrostatic transmission for such a transaxle includes a hydraulic pump,which is driven by an input shaft from the engine output by means of thebelt and pulley arrangement, and a hydraulic motor, both pump and motorare mounted on a center section located inside the transaxle housing.Rotation of the pump by an input shaft creates axial motion of the pumppistons during periods when the pistons are operating against aninclined thrust or swash-plate. The fluid flow thus created by thereciprocating axial motion of the pistons is channelled via porting andpassages in the center section to the hydraulic motor, with the effectthat the incoming fluid causes the pistons of the motor to reciprocateand create a turning moment that causes rotation of the hydraulic motor.The hydraulic motor in turn has an output shaft which drives thevehicle's axles through speed-reducing gears and a mechanicaldifferential. Examples of such hydrostatic transaxles are shown in thefollowing patents: U.S. Pat. Nos. 5,090,949; 5,473,964 and 5,501,640.

All three references use an axial piston swash-plate pump and motorrespectively engaged to a center section which is located within atwo-shell housing structure. The main purpose of the center section isto provide a fluid link between the pump and the motor and allowing thetransmission of hydraulic power. Patents '964 and '640 in the names ofOkada and Hauser respectively, teach the use of an input shaft drivenpump where the swash-plate lies adjacent to the upper housing. Thiscontrasts with the disclosure in patent '949 which teaches the use ofbevel gears for connecting the input-shaft to the pump and wherein thisexample of prior art, the swash-plate of the pump lies directly acrossboth the upper and lower housings of the transaxle.

The center section shown in all three above references require numerousmachining operations to prepare the initial raw casting to be ready foruse. For instance: drilling some or all of the internal flow passagesand arranging retaining means so that plugs/valves and such like can besubsequently fitted to close off the flow circuit; making good two ofthe faces which provide the fluid coupling means for the pump and motor,and when required, for the subsequent attachment of the valve-plates;preparation of mounting surfaces for attaching the center section to thehousing structure. Furthermore, the upper transaxle housing aluminiumdiecasting itself requires a number of machining operations before itcan be used such as the provision for the shaft bearing and seal as wellas hole or holes and seals for the control lever and various associatedlinkages.

As a general rule, the more machining operations required in the uppertransaxle housing casting as well as the more complex operationsrequired in the center section casting, the greater the cost ofmanufacture of the complete hydrostatic transaxle. Therefore thereduction in the number of such machining operations and by groupingthem into one rather than two components would save expense.

Although only shown in the '949 patent, almost all hydrostatictransaxles make use of a cooling fan mounted to the input drive shaft inan attempt to help prevent the internal components and fluid fromoverheating. However, the prior art teaches a center section whichalthough attached in some manner to the interior of the housing, it isstill essentially a separate entity from the transaxle housing. As aresult, effective cooling of the fluid passing through the passages inthe center section that connect the pump and motor together is hinderedas the fluid surrounding the center section acts as a insulator to slowdown the rate of heat transfer from the power transmission fluid in saidpassages to the surrounding housing radiator.

The amount of heat able to be radiated away from the transaxle housingexterior to the surrounding environment is of course greatly enhancedover that region on the boundary of the transaxle housing that liesdirectly in the path of the air flow from the cooling fan. However, itis apparent that although the fluid inside the housing nearest thatregion where the fan is operating is being cooled, fluid elsewhere maystill remain at very high temperature. Perhaps more importantly, as thefluid circulating between the pump and motor in the fluid passages inthe center section becomes extremely hot during operation, especiallywhen the unit is heavily loaded and used in a high ambient temperatureenvironment, the resulting drop of operating efficiency due todecreasing fluid viscosity and a corresponding increase in fluid leakagelosses can be a concern with the prior art.

This problem exists because the attendant power losses associated withsuch close coupled pump and motor combinations produce a lot of unwantedheat due to the rapid fluid compression/decompression cycles and generalfriction between the sliding surfaces. Such losses causes the fluidcirculating between the pump and motor through the center section tobecome extremely hot, and because the prior art teaches a transaxlehousing structure whereby the internal fluid reservoir completely oralmost completely surrounds and insulates the center section, theseprior solutions are not conducive to the promotion of most effectivecooling for the circulating fluid in the centre section flowing in aclosed-loop circuit between the pump and motor. This limitation occursbecause the bulk of the heat accumulating in the center section can onlybe transferred by conduction to the surrounding hydraulic fluid and thenthrough the fluid itself to reach the boundary walls of the housingsurrounding the fluid chamber from where it can be radiated away to thesurroundings. The remove of unwanted heat from the center sectionconsequently takes time.

Therefore in these prior devices where the center section is effectivelyinsulated by the surrounding hydraulic fluid, the delay in the transferof unwanted heat out of the transaxle may on occasion result in thefluid of the hydrostatic transmission becoming overheated with the riskthat the operational life of the fluid is shortened or that thelubricating properties of the fluid deteriorates to the extent thatthreatens the useful operational life of the hydrostatic transaxle.

SUMMARY OF THE INVENTION

An object of the invention is to eliminate the need to use aconventional centre section in the hydrostatic transaxle product.

A further object of the invention is to provide a hydrostatic transaxlein which most or all the machining can be carried out in one housingmember compared to the prior art where such operations need to becarried out in both the housing as well as the center section.

A still further object of the invention is the grouping of the fluidcoupling surfaces for the pump and motor as well as the heat dissipatingmeans on the exterior surfaces on a single housing component, includingfluid passages arranged in its interior with bearings, seals, controlshafts supported in machined pockets.

A further object of the invention is to improve the cooling of theoperating fluid circulating between the pump and motor. With thisinvention, fan cooling of the housing very effective in lowering thetemperature of the hydrostatic transmission fluid as it circulates inthe closed-loop circuit between the pump and motor as much of the heatis conducted directly into the material of the housing in the areadirectly under the path of the air from the cooling fan.

What is needed in thee art is a compact hydrostatic transaxle forvertical input shaft installations where the amount of machining neededto be carried out is consigned to one rather than two or three maincomponent members, preferable in a manner that would allow both theupper and the lower transaxle housing elements or shells for thehydrostatic transaxle to be used in an as cast or supplied state. Whatis further needed in the art is improved cooling for such devicesallowing an extension in the operation duty cycle.

In one form thereof, the hydrostatic transaxle of the inventioncomprises an axle assembly with a housing having an internal chamber foran internally disposed hydrostatic transmission and forming an integralpart of said hydrostatic transmission, said hydrostatic transmissioncomprising a variable-displacement hydraulic pump and afixed-displacement hydraulic motor; axle shafts rotatably supported insaid housing and differential gearing means supported by said housingdrivingly connected between said hydraulic motor and said axle shafts;said housing comprising first, second and third housing members andwhere said first and second housing members are joined together on afirst parting plane and where said second and third housing members arejoined on a second parting plane and wherein said first housing memberincludes internal fluid passages arranged to fluidly connect saidhydraulic pump to said hydraulic motor.

The above mentioned and other novel features and objects of theinvention, and the manner of attaining them, may be performed in variousways and will now be described by way of examples with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external view from one side of the hydrostatic transaxleaccording to the invention.

FIG. 2 is a plan view of the hydrostatic transaxle of FIG. 1 along thesection line I—I.

FIG. 3 is a sectioned view on line II—II of FIG. 2 showing thehydrostatic transmission.

FIG. 4 is a further sectioned view on line III—III of FIG. 2.

FIG. 5 is a view of an alternative housing construction for thehydrostatic transaxle of FIG. 1.

FIG. 6 is a part sectional view along line IV—IV of FIG. 5.

FIG. 7 depicts external control linkages for use with the alternativehousing construction of FIG. 5.

FIG. 8 is a part sectional view along line V—V of FIG. 7.

FIG. 9 is a sectioned view taken at line VI—VI of FIG. 8.

FIG. 10 depict various forms of fluid valves for use in the alternativehousing construction.

DETAILED DESCRIPTION OF THE FIRST EMBODIMENT OF THE INVENTION

The first embodiment shown in FIGS. 1 to 4, the outer housing structureof the hydrostatic transaxle depicted by arrow 1 is partially defined byan upper transaxle housing element 2 and a lower transaxle housingelement 3 which are joined together at a junction surface arrangedpreferably such that the parting-plane 5 is coincident with the outputaxle shafts 7, 8. Parting-plane can be called the second parting-planein this invention. A liquid gasket seal is applied to the junctionsurface at the parting plane 5 prior to the two transaxle housingselement 2, 3 being secured together by a plurality of bolts or screws10. As shown in FIGS. 3 & 4, upper transaxle housing element 2 isarranged to have opening 12 through which housing member 13 passes andwhich is then secured in-place by a plurality of fastening screws 15. Aseal ring or liquid gasket seal is applied to the junction surface 16between housings members 2, 13 next to opening 12. Junction surface 16can be called the first parting-plane in this invention. All threehousing members 2, 3, 13 when attached together form the surroundingboundary for an internal chamber 17. Within internal chamber 17,internal elements of the hydrostatic transaxle are located therein suchas the hydraulic pump, hydraulic motor, speed reducing gears andmechanical differential. Although internal chamber 17 may be dividedinto separate chambers for purposes of segregating the hydrostatictransmission from the mechanical gearing (and when required a mechanicaldifferential), the embodiment here illustrates the use of a commonchamber 17 for all such components.

Housing member 13 may preferably be cast as one-piece in eitheraluminium allow or iron/steel. If the piece is cast as a pressurediecasting, many features can be detailed with sufficient accuracy suchthat many secondary machining operations can be thus avoided.

As a portion of housing member 13 containing within it the fluidpassages connecting the pump and motor is exposed to the outerenvironment, porosity in the casting could led to fluid leakage.However, the possibility of such fluid seepage through the material ofthe housing member in the present invention can be easily overcome in atleast one way, for instance, through the impregnating of that portion ofthe housing member that would protrude through the opening in the upperhousing member of the transaxle with a resin that prevents leakageoccurring.

Housing member 13 contains within its structure fluid passages 20, 21,22, 23 that connect together the respective fluid coupling surfacesotherwise known as valve-faces, between the pump and motor. Fluidpassages 20, 21, 22, 23 may be cored in the housing casting 13 ormachined. Plugs 25 are used to blank off the ends of passages 20, 21.Although in practice it has been known to cast valve faces in aluminiumalloy to provide a running surface for a cylinder-barrel, most commonpractise these days is to mount separate valve-plates on whichrespective cylinder-barrels can run against. Most often, such separatevalve-plates are surface hardened so that the sealing surfaces mayoperate for many thousands of hours without suffering from undue wearthat would result in a loss in volumetric efficiency of the hydrostaticunit. In this respect, the faces shown as 28, 29 on the housing member13 may be cast with countersunk register shown as 30, 31 respectively,into which respective valve plates 33, 34 are located. However, theinvention would permit the valve faces to be detailed and formeddirectly on the surface of the subsidiary housing 13 if desired.

Check-valves are included in both respective pairs of passages 20, 22and 21, 23 to allow the admittance of makeup fluid in order that thehydrostatic transmissions can recover any fluid loss during operationbecause of leakage. In the present invention, the check-valves shown as37 have been placed near the lowest position in the downwardly extendingportion shown as 40 of the housing member 13 such that balls 42 of thecheck-valves 37 can fall onto their respective seats 44 by the influenceof gravity when not activated by suction pressure. Although not shown,the linkage connecting the swash-plate of the pump to the externallyprotruding control-lever of the hydrostatic transaxle may be arranged sothat when the pump is at or near neutral, the linkage can act in amanner to off seat the balls, thereby proving the hydrostatictransmission with a more positive neutral point.

The respective cylinder-barrels 60, 61 of the hydrostatic-transmissionpump and motor are mounted perpendicular to one another such that therotating axis of the pump cylinder-barrel 60 is vertical and arrangedparallel and coaxial with respect to the input-drive shaft 62 whereasthe rotating axis of the motor cylinder-barrel 61 is parallel withrespect to the rotating axis of the axle-shafts 7, 8. The input driveshaft 62 is supported by a bearing 63 in the housing element 13 and aseal 64 is used to prevent fluid in the internal chamber 17 fromescaping. Shaft 62 extends downwards and is provided with a spline 65which connects with the pump cylinder-barrel 60. Shaft 62 extendsfurther and passes through the center of the swash-plate 70 to befurther supported by means of a bearing 72 in lower housing member 3.

Bearing 72 is supported in blind hole 71 which ideally may be sized bythe die-caster thereby eliminating any need to machine lower housingmember by the transaxle builder although in practice. Alternatively,hole 71 could be cast as a through hole and in this case, an extra sealwould then be needed. Bearing 72 may be of the self-aligning type inwhich case a plain journal bearing may be added to the designpositioned, this bearing being positioned just adjacent to valve-plate33 to act between bearings 63, 72 for providing additional support fordrive-shaft 62.

To overcome any misalignment between the bearing 63 in housing member 13and bearing 72 in the lower housing member 3, it is proposed that duringassembly of the hydrostatic transaxle, housing member 13 is only looselyattached by screws 15 until all the components have been assembled inplace. Then once input shaft 62 is located into bearing 72, thisessentially controls the true position of the housing 13 relative tohousing members 2, 3 as the bearing 63 supported input shaft 62 isthereby in correct alignment with bearings 72. At that time, screws 15can be tightened so that housing elements 2, 13 are thereby lockedtogether.

The cylinder-barrel 60 of the pump is provided with a plurality of axialcylinder-bores 75, each bore 75 containing a respective piston 76 andwhere each piston 76 is being axially urged outwards by a spring (notshown) located behind the piston 76 in the bore 75. The outer end of thepiston 76 is generally domed-shaped to be operatively connected to anadjacent operating surface 80 of swash-plate 70 by the bias produced bysprings. The action of the springs behind each of the pistons 76produces a counter reaction which loads the cylinder-barrel 60 againstthe operating surface 81 of the adjacent valve-plate 33. Eachcylinder-bore 75 has a port 77 so arranged to communicate in sequencewith a pair of arcuate-shaped ports (although not visible they areidentical to those arcuate-shaped ports 98, 99 shown for the motor)provided on the valve-plate 33, and where such arcuate-shaped ports arein fluid connection with fluid passages 20, 21 in housing member 13.

Swash-plate 70 is so arranged for the pump that its inclination anglecan be varied in both directions from its neutral or zero-inclinationpoint. The swash-plate 70 is seated on a part-cylindrical bearing 88provided in the interior of the lower housing member 3, and connected bylinkages (not shown) to a control-shaft 90 which protrudes from housingmember 2 as shown in FIG. 1. Rotary movement of the control-shaft 90causes the swash-plate 70 to incline in angle in respect of the strokingaxis of the pistons 76, and thereby the stroke of the pistons 76 ischanged. The amount of piston 76 stroke determines the amount of fluidis placed in the cylinder-bore 75 per each single rotation of thecylinder-barrel 60, and hence the swept volume of the pump can bechanged by altering the amount of piston stroke so that the amount offluid delivered to the hydraulic motor is precisely controlled.

The cylinder-barrel 61 of the motor is almost in all respects identicalto that of the pump, and carries a series of pistons 93 which areoperatively connected to the operational surface 94 of thrust plate 95.Each piston 93 is housed in its respective cylinder-bore 96 providedwithin cylinder-barrel 61, and arranged so that each cylinder-bore 96can communicate in sequence with respective arcuate-shaped ports shownas 98, 99 provided in the valve-plate 34 by means of its respective port97. Fluid passages 23, 23 in the housing member 13 are arranged to be influid connection with respective arcuate shaped ports 98, 99 in thevalve-plate 34 of the motor by means of respective linking ducts 91, 92.Although not shown, a spring is positioned within each of thecylinder-bores 96 to engage with its respective piston.

In the case of the motor, the thrust-plate 95 is depicted in its mostoften used position which is permanently inclined with respect of theaxis of pistons 93. The thrust-plate 95 is supported on a insert 100that is held to the housing member 2, 3 either by fastening mens such asscrews or preferably by being pinched tight between adjacent walls asshown 101, 102 in housing member 2, 3. Because the inclination angle ofthe thrust-plate 95 always remains at an angle during the operation ofthe device, the piston 93 stroke in the motor remain constant. As thefluid received from the pistons 76 of the pump can be changed by theaction of using control-shaft 90, the rotation of the motor can be ineither direction. The cylinder-barrel 61 is mechanically engaged byspline 105 to shaft 106 and thus rotation of the cylinder-barrel 61causes rotation of shaft 106. As fluid enters the motor from the arcuateshaped ports 98, 99 in the valve-plate 34, the fluid entering thecylinders of the pistons 93 causes the pistons 93 to move axiallyoutwards and because the reaction on the thrust-plate 95 to the pistonmovement is not co-axial with the longitudinal axis of the pistons 93,an angular driving moment is created on the cylinder-barrel 61 which iscaused to revolve. Therefore rotation of the cylinder-barrel 61 andshaft 106 is transmitted through the speed reducing gears to thedifferential and axle output shafts 7, 8 of the hydrostatic transaxle 1which in the case of a vehicle application such as lawn tractor, areattached the drive wheels of the vehicle.

Shaft 106 is supported by bearings 110, 111, bearing 110 being locatedin the downwards extending portion 40 of subsidiary housing member 13,whereas bearing 111 is located in a pocket 112 arranged between housingmember 2, 3. The shaft 106 may protrude from the transaxle so that aconventional disc parking brake can be attached. A seal 114 is alsoprovided to surround the shaft 106 in order to prevents fluid seepingout of the internal chamber 17.

A gear 120 fixed to shaft 106 is in mesh with gear 123 which is fixed tointermediary-shaft 125. Intermediary shaft 125 is supported by bearings127, 128 in similar manner to that described for the shaft 106. Gear 130fixed to intermediary-shaft 125 is in mesh with the ring-gear 131 of thedifferential-assembly.

The ring-gear 131 of the differential-assembly has bevel gears shown as132, 133, 134, 135 so that power can be transmitted from thedifferential-assembly to the axle-shafts 7, 8 of the transaxle 1 asknown to those skilled in the art. The inclusion of a differentialassembly is important as it allows normal differentiation between theleft and right drive wheels of the vehicle and helps prevent lawn damageespecially when tight turns are undertaken However, there areapplications where no such differentialled action is required, and inthese instances, a single axle shaft may be used instead of the two asshown in this embodiment. In the case of a single axle shaft, this shaftcan be arranged to extend outwardly on one or both sides form thehousing.

By means of appropriate selection or adjustment of the inclination ofthe swash-plate 70 of the pump by means of the control-shaft 90 and theintermediary linkage, the hydrostatic transmission ratio is altered.Rotation of the input-shaft 62 causes cylinder-barrel 60 to rotate andresults in reciprocation of pistons 76. Fluid is then delivered frompassages 20, 22 or 21, 23 (depending on which direction of low occursfrom the pump) in the housing member 13 and enters the cylinder-barrel61 of the motor and causes the pistons 93 to reciprocate in theirrespective bores by way of their angle of attack against the inclinedthrust-plate 95. A side force is created by the pistons 93 on the wallof each bore in those bores subjected to pressurised fluid, causingrotation of the motor cylinder-barrel 61 about its longitudinal orcentral axis for rotation. The cylinder-barrel 61 in turn rotates shaft106 and mechanical power is transmitted through gears 10, 123, 130 tothe ring gear 131 of the differential. Bevel gearing of the differentialthen determines the respective speeds of the axle output shafts 7, 8that drive the wheels of the vehicle.

Detailed Description of the Second Embodiment of the Invention

As the second embodiment differs in only one main respect from the firstembodiment, description is only necessary to show the main points ofdifferences. As the internal components are identical to those describedfor the first embodiment, for convenience, most that are here numberedwill carry the same reference numeral as for the first embodiment.

Essentially as shown in FIGS. 5 to 9, the housing member 200 containingwithin the pairs of fluid passages 201, 203 and 202, 204 that are usedin linking the pump to the motor, is attached to an exterior junctionsurface (first parting-plane) shown as 210 provided on the upper surfaceof housing member 212. Housing element 212 is provided with an opening215 which acts as the register for housing member 200 and wherefastening screws 217 are used to secure housing members 200, 212together. Housing member 212 combine with the lower transaxle housingmember 213 to form an internal chamber 220.

Housing member 200 is provided with a face surface 221 for thevalve-plate of the pump, and on the downwards extending portion 205 isprovided with face surface 222 for the valve-plate 34 of the motor.Check-valves show as 225 are included for each passages 203, 204respectively, and plugs 226, 227 as shown in FIG. 9, are used to closethe ends of passages 201, 202. Plugs 226, 227 do not need to be threadedin passages 201, 202 as they are prevented from being expelled becauseof adjacent wall formed by the opening 215 in housing member 212.

This embodiment also contrasts with the first embodiment in thatsubstantially more cooling fins shown as 233 can be included on the topsurface 234 of housing member 200, thereby providing more efficient andeffective cooling of the fluid passing through passage 201, 202, 203,204 between the pump and motor.

Although the present invention will still allow some of the heat to betransferred out from the transaxle by conduction through the hydraulicfluid medium to the outer housing in a similar manner as used in theprior art devices, the total cooling effect is enhanced because the mostimportant area to be cooled, namely the fluid path between pump andmotor, is directly adjacent that portion in the housing on which thecooling fan is most effective. Consequently, the hydrostatic transaxlecan be operated for longer periods at rated loads with less risk ofoverheating the power transmission fluid.

A control-shaft 251 is journalled at 252 in the housing member 200.Having the control shaft located in the housing member containing theinternal fluid passages can also be performed in the first embodiment ofthe invention if so desired.

Rotary vent valve 260 shown in FIGS. 7 to 10 may be used to provided a“wider band neutral” effect for the hydrostatic transmission, and aswell as if desired, a “freewheel” or fluid dumping effect. Essentially,the valve 260 is supported in bore 261 in housing member 200 and isprovided with a fluid short-circuit shown as small passage 263 which,when the swash-plate of the pump is near its zero inclination angle,passage can connect with two passages shown as 265, 266 in housingmember 200 which are in communication with fluid passages 201, 203 and202, 204 respectively. As shown, any fluid released by the pump when theswash-plate has a small inclination angle can divert through passage 263rather than flow to the hydraulic motor where it would act in causingthe motor assembly to rotate. FIG. 7 shows how valve 260 can be linkedto control-shaft 251 by means of linkages 270, 271, 272 so that themovement in position of passage 263 occurs as control-shaft 251 is movedby the operator of the vehicle. By varying in the span or arm length ofthe linkages, it is possible to obtain varying characteristics from the“wider band neutral” to suit each particular application. Valve 260 isshown as the left-hand illustration in FIG. 10 and shows hidden passage269 which is perpendicular to passage 263. Disconnection of linkage 270with valve 260 would allow the valve 260 to be rotated through ninetydegrees to provide a large flow short-circuit between passages 265, 266as hidden passage 269 becomes a short-circuit for the fluid. Valve 280shows a modification whereby grooves 281, 282 are provided for “O” ringtype seals that surround passage 263. Valve 290 illustrates a furthermodification where the valve can be axially lifted so that passage 263is no-longer in connection with passages 265, 266 in housing member 200,so that in the lifted position, slot 291 is open to passages 265, 266 toshort-circuit fluid into internal chamber 220. Thus the degree of flowrestriction caused by fluid having to pass through the restrictedpassage 263 is overrided, and the vehicle can be easy pushed withouthaving to first start the engine. A further advantage of having the ventvalve in the housing element is that adjustments to the operatingcondition of the hydrostatic transaxle can be made without disassemblyof the entire unit.

Although not shown or described in either of the embodiments of theinvention, a further modification falling within the scope of thepresent invention would be to rearrange lower housing member shown as 3in the area about the swash-plate 70 and bearing 72 so that a fourthhousing member could be used. The fourth housing member would beprovided with a hole for bearing 72 and a part-cylindricalsupport-surface for the swash-plate 70. The fourth housing member couldbe attached and secured to the inside interior wall of housing element 3or alternatively, be attached to the outside of housing element 3 on amounting face and arranged that part of it extends through an openingprovided the housing member 3. In this manner, the housing membercontaining the fluid passages can further be attached to the fourthhousing element by means of stays or studs which are arranged to passthrough the top exterior to the bottom exterior which would stiffen thecomplete housing structure of the hydrostatic transaxle.

Perhaps for certain applications, there may be advantage in substitutingthe fixed-displacement axial piston swash-plate hydraulic motor withthat of another type. For instance, a fixed-displacement external gearedhydraulic motor could be used instead and where the gears of the motorin this instance would be journalled in bores provided in the samehousing member containing the fluid passages linking the pump to themotor.

A charge and/or power take-off auxiliary pump, preferably of the gerotorinternal gear type may also be disposed in the same housing member thatcontains the fluid passages linking the pump to the motor of thehydrostatic transmission. The gerotor pump being driven by the inputdrive shaft and having fluid passages and valves arranged in the samehousing member to suit the needs of the application. A furtheradvantages in this arrangement would allow the pressure setting of thegerotor pump to be adjusted easily as the pressure relief-valve wouldhave an external adjustment; the fan cooling effect of the presentinvention would help keep the fluid delivered by the gerotor pump to bekept as cool as possible; all the external connections can be arrangedto be near the top of the transaxle thereby minimising the changes ofbeing damaged.

In accordance with the patent statutes, we have described the principlesof construction and operation of our invention, and while we haveendeavoured to set forth the best embodiments thereof, we desire to haveit understood that obvious changes may be made within the scope of thefollowing claims without departing from the spirit of our invention.

We claim as follows:
 1. An integrated hydrostatic transaxle comprising ahousing having an internal chamber for an internally disposedhydrostatic transmission; said hydrostatic transmission comprising avariable-displacement axial piston pump having a set of axial pistonsdisposed in a rotary pump cylinder barrel and bearing on an adjacentswash plate, and a fixed-displacement axial piston motor having a set ofaxial pistons disposed in a rotary motor cylinder barrel and bearing onan adjacent inclined thrust plate; an input shaft having a vertical axisof rotation rotatably supported in said housing and engaging said rotarypump cylinder barrel; a cooling fan disposed on said input shaft outsidesaid housing and having a plurality of blades for moving air towardssaid housing; a motor output shaft having a horizontal axis of rotationrotatably supported in said housing and engaging said rotary motorcylinder barrel; speed reducing gearing in said housing and drivinglyconnected to said motor output shaft; a mechanical differential in saidhousing and drivingly connected to said speed reducing gearing; a pairof axle shafts rotatably supported in said housing drivingly connectedto said mechanical differential and extending outwardly from saidhousing; said housing having a lower transaxle housing element and anupper transaxle housing element joined together along a horizontalparting plane, said upper housing element including a transverse channelopening on said parting plane to receive said pair of axle shafts, andan interior junction surface on which is mounted a casting separate fromsaid lower transaxle housing element containing internal fluid passages;first and second fluid coupling surfaces inside said housing andhydraulically interconnected by said internal fluid passages, said pumpcylinder barrel engaging the first fluid coupling surface and said motorcylinder barrel engaging the second fluid coupling surface, said firstfluid coupling surface being disposed in a first horizontal plane andsaid motor output shaft having a horizontal axis of rotation disposed ina second horizontal plane, wherein said first horizontal plane issituated nearer said cooling fan than said second horizontal plane. 2.The integrated hydrostatic transaxle according to claim 1 wherein thelongitudinal axis of rotation of said pair of axle shafts is disposedparallel to said parting plane.
 3. The integrated hydrostatic transaxleaccording to claim 2 wherein said interior junction surface is offsetfrom said parting plane and disposed in a horizontal plane to be nearersaid cooling fan than said second horizontal plane.
 4. The integratedhydrostatic transaxle according to claim 3 wherein the mounting of saidcasting to said interior junction surface conducts heat away from saidinternal fluid passages exteriorly of said housing by the action of saidcooling fan.
 5. The integrated hydrostatic transaxle according to claim4 wherein check valves are disposed in said internal fluid passages andwhere at least one of said check valves is arranged to reside at anelevation below said second horizontal plane.
 6. The integratedhydrostatic transaxle according to claim 5 wherein the elevation of saidat least one of said check valves is lower than said first horizontalplane and said horizontal plane of said interior junction surface. 7.The integrated hydrostatic transaxle according to claim 6, furthercomprising an internal fluid releasing device to allow a release ofhigh-pressure fluid from whichever of said internal fluid passagescontains fluid under a higher pressure once the inclination angle ofsaid swash plate of said hydraulic pump is decreased to a position setin accordance with a wider band neutral condition.
 8. The integratedhydrostatic transaxle according to claim 7, further comprising adjacentbulkhead walls formed on said lower transaxle housing element and saidupper transaxle housing element, respectively, said adjacent bulkheadwalls serving to segregate an inner fluid reservoir surrounding saidhydrostatic transmission from an inner fluid reservoir surrounding saidspeed reducing gearing and said mechanical differential.
 9. Theintegrated hydrostatic transaxle according to claim 7, furthercomprising adjacent bulkhead walls formed on said lower transaxlehousing element and said upper transaxle housing element, respectively,said adjacent bulkhead walls serving to separate an inner fluidreservoir surrounding said hydrostatic transmission from an inner fluidreservoir surrounding said speed reducing gearing and said mechanicaldifferential.
 10. The integrated hydrostatic transaxle according toclaim 9, further comprising a transverse channel in said lower transaxlehousing element and opening on said parting plane for receiving saidpair of axle shafts, and wherein said longitudinal axis of said axleshafts is coincident with said parting plane.
 11. The integratedhydrostatic transaxle according to claim 10 wherein said secondhorizontal plane is coincident with said parting plane.
 12. Theintegrated hydrostatic transaxle according to claim 11 wherein a portionof said internal fluid passages are at an elevation above said firsthorizontal plane, and wherein said swash plate is at an elevation belowsaid first horizontal plane.
 13. The integrated hydrostatic transaxleaccording to claim 12 wherein said casting includes an integral spigotportion to carry a rotary seal and wherein said casting remainsinteriorly disposed in said housing except for said spigot portion, saidspigot portion extending through an opening provided in said uppertransaxle housing element and exposed exteriorly of said housing. 14.The integrated hydrostatic transaxle according to claim 13 wherein saidinput shaft is rotatably supported by a pair of bearings and wherein onebearing is disposed in said lower transaxle housing element and theother bearing is disposed adjacent said rotary seal in said spigotportion.
 15. The integrated hydrostatic transaxle according to claim 1,wherein said casting is a further housing element.
 16. An integratedhydrostatic transaxle comprising a housing having an internal chamberfor an internally disposed hydrostatic transmission; said hydrostatictransmission comprising a variable-displacement axial piston pump havinga set of axial pistons disposed in a rotary pump cylinder barrel andbearing on an adjacent swash plate, and a fixed-displacement axialpiston motor having a set of axial pistons disposed in a rotary motorcylinder barrel and bearing on an adjacent inclined thrust plate; aninput shaft having a vertical axis of rotation rotatably supported insaid housing and engaging said rotary pump cylinder barrel; a coolingfan disposed on said input shaft outside said housing and having aplurality of blades for moving air towards said housing; a motor outputshaft having a horizontal axis of rotation rotatably supported in saidhousing and engaging said rotary motor cylinder barrel; speed reducinggearing in said housing and drivingly connected to said motor outputshaft; a mechanical differential in said housing and drivingly connectedto said speed reducing gearing; a pair of axle shafts rotatablysupported in said housing drivingly connected to said mechanicaldifferential and extending outwardly from said housing; said housinghaving a first transaxle housing element and a second transaxle housingelement joined together at a parting plane and where one of thetransaxle housing elements includes an interior junction surface onwhich is mounted a casting containing internal fluid passages; first andsecond fluid coupling surfaces inside said housing and hydraulicallyinterconnected by said internal fluid passages, said pump cylinderbarrel engaging the first fluid coupling surface and said motor cylinderbarrel engaging the second fluid coupling surface, said first fluidcoupling surface being disposed in a first horizontal plane separatefrom said second transaxle housing element.
 17. The integratedhydrostatic transaxle according to claim 16 wherein said housingsupports said casting at a location radially adjacent said second fluidcoupling surface.
 18. The integrated hydrostatic transaxle according toclaim 17, further comprising a transverse channel in said firsttransaxle housing element and opening on said parting plane forreceiving said pair of axle shafts and wherein the longitudinal axis ofrotation of said pair of axle shafts is disposed parallel to saidparting plane.
 19. The integrated hydrostatic transaxle according toclaim 18, further comprising a transverse channel in said secondtransaxle housing element and opening on said parting plane forreceiving said pair of axle shafts and wherein said longitudinal axis ofrotation of said pair of axle shafts is coincident with said partingplane.
 20. The integrated hydrostatic transaxle according to claim 17wherein said housing includes internal bulkhead walls, said bulkheadwalls separating an inner fluid reservoir surrounding said hydrostatictransmission from an inner fluid reservoir surrounding said speedreducing gearing and said mechanical differential.
 21. The integratedhydrostatic transaxle according to claim 17 wherein said housingincludes internal bulkhead walls, said bulkhead walls segregating aninner fluid reservoir surrounding said hydrostatic transmission from aninner fluid reservoir surrounding said speed reducing gearing and saidmechanical differential.
 22. The integrated hydrostatic transaxleaccording to claim 17 wherein said support for said casting conductsheat away from said internal fluid passages exteriorly said housing bythe action of said cooling fan.
 23. The integrated hydrostatic transaxleaccording to claim 22 wherein said interior junction surface is disposedat an elevation in said housing closer to said cooling fan than theelevation of said second horizontal plane.
 24. The integratedhydrostatic transaxle according to claim 23 wherein said parting planeat the boundary between said first and second transaxle housing elementsis arranged to be coincident with the longitudinal axis of said pair ofaxle shafts.
 25. The integrated hydrostatic transaxle according to claim24 wherein a portion of said internal fluid passages are at an elevationabove said first horizontal plane, and wherein said swash plate is at anelevation below said first horizontal plane.
 26. The integratedhydrostatic transaxle according to claim 25 wherein said casting saidincludes an integral spigot portion to carry a rotary seal and whereinsaid casting remains interiorly disposed in said housing except for saidspigot portion, said spigot portion extending through an openingprovided in said housing and exposed exteriorly of said housing.
 27. Theintegrated hydrostatic transaxle according to claim 26 wherein saidinput shaft is rotatably supported by a pair of bearings and wherein onebearing is disposed in said housing and the other bearing is disposedadjacent said rotary seal in said spigot portion.
 28. The integratedhydrostatic transaxle according to claim 16, wherein said casting is afurther housing element.
 29. An integrated hydrostatic transaxlecomprising a housing having an internal chamber for an internallydisposed hydrostatic transmission; said hydrostatic transmissioncomprising a variable-displacement axial piston pump having a set ofaxial pistons disposed in a rotary pump cylinder barrel and bearing onan adjacent swash plate, and a fixed-displacement axial piston motorhaving a set of axial pistons disposed in a rotary motor cylinder barreland bearing on an adjacent inclined thrust plate; a motor output shafthaving a horizontal axis of rotation rotatably supported in said housingand engaging said rotary motor cylinder barrel; speed reducing gearingin said housing and at least one axle shaft rotatably supported in saidhousing and extending outwardly from said housing; said motor outputshaft and said at least one axle shaft being operatively interconnectedvia said speed reducing gearing; said housing having a lower transaxlehousing element and an upper transaxle housing element joined togetheralong a horizontal parting plane, said upper housing element including atransverse channel opening on said parting plane to receive said atleast one axle shaft, and an interior junction surface on which ismounted a casting separate from said lower transaxle housing elementcontaining internal fluid passages; first and second fluid couplingsurfaces inside said housing and hydraulically interconnected by saidinternal fluid passages, said pump cylinder barrel engaging the firstfluid coupling surface and said motor cylinder barrel engaging thesecond fluid coupling surface, said first fluid coupling surface beingdisposed in a first horizontal plane and said motor output shaft havinga horizontal axis of rotation disposed in a second horizontal plane,wherein said first horizontal plane is situated nearer said interiorjunction surface than said second horizontal plane.
 30. The integratedhydrostatic transaxle according to claim 29 wherein the longitudinalaxis of rotation of said at least one axle shaft is disposed parallel tosaid parting plane.
 31. The integrated hydrostatic transaxle accordingto claim 30 wherein said longitudinal axis of said at least one axleshaft is arranged to lie coincident with said parting plane.
 32. Theintegrated hydrostatic transaxle according to claim 30 and including aninput shaft having a vertical axis of rotation rotatably supported insaid housing and a cooling fan disposed on said input shaft outside saidhousing and having a plurality of blades for moving air towards saidhousing, and where said interior junction surface is disposed on a thirdhorizontal plane offset from said parting plane, said third horizontalplane is situated nearer said cooling fan than said second horizontalplane.
 33. The integrated hydrostatic transaxle according to claim 32wherein the mounting of said casting to said interior junction surfaceconducts heat away from said internal fluid passages exteriorly of saidhousing by the action of said cooling fan.
 34. The integratedhydrostatic transaxle according to claim 33 wherein check valves aredisposed in said internal fluid passages and where at least one of saidcheck valves is arranged to reside at an elevation below said secondhorizontal plane.
 35. The integrated hydrostatic transaxle according toclaim 34 wherein said at least one of said check valves is furtherarranged to reside at an elevation lower than the respective elevationsof said first horizontal plane and said second horizontal plane.
 36. Theintegrated hydrostatic transaxle according to claim 33, furthercomprising an internal fluid releasing device to allow a release ofhigh-pressure fluid from whichever of said internal fluid passagescontains fluid under a higher pressure once the inclination angle ofsaid swash plate of said hydraulic pump is decreased to a position setin accordance with a wider band neutral condition.
 37. An axle drivingassembly according to claim 33 and, further comprising adjacent bulkheadwalls formed on said lower transaxle housing element and said uppertransaxle housing element, respectively, said adjacent bulkhead wallsserving to segregate an inner fluid reservoir surrounding saidhydrostatic transmission from an inner fluid reservoir surrounding saidspeed reducing gearing and said mechanical differential.
 38. Theintegrated hydrostatic transaxle according to claim 33, furthercomprising adjacent bulkhead walls formed on said lower transaxlehousing element and said upper transaxle housing element, respectively,said adjacent bulkhead walls serving to separate an inner fluidreservoir surrounding said hydrostatic transmission from an inner fluidreservoir surrounding said speed reducing gearing and said mechanicaldifferential.
 39. The integrated hydrostatic transaxle according toclaim 38 wherein said second plane is coincident with said partingplane.
 40. The integrated hydrostatic transaxle according to claim 39wherein said input shaft is operatively connected to said pump cylinderbarrel to rotate at same speed.
 41. The integrated hydrostatic transaxleaccording to claim 40 wherein a portion of said internal fluid passagesare at an elevation above said first horizontal plane, and wherein saidswash plate is at an elevation below said first horizontal plane. 42.The integrated hydrostatic transaxle according to claim 41 wherein saidcasting includes an integral spigot portion to carry a rotary seal andwherein said casting remains interiorly disposed in said housing exceptfor said spigot portion, said spigot portion extending through anopening provided in said upper transaxle housing element and exposedexteriorly of said housing.
 43. The integrated hydrostatic transaxleaccording to claim 42 wherein said input shaft is rotatably supported bya pair of bearings and wherein one bearing is disposed in said lowertransaxle housing element and the other bearing is disposed adjacentsaid rotary seal in said spigot portion.
 44. The integrated hydrostatictransaxle according to claim 32, wherein said casting is a furtherhousing element.
 45. The integrated hydrostatic transaxle according toclaim 43, further comprising a transverse channel in said lowertransaxle housing element opening on said parting plane for receivingsaid at least one axle shaft, and wherein said longitudinal axis of saidat least one axle shaft is coincident with said parting plane.
 46. Theintegrated hydrostatic transaxle according to claim 45, furthercomprising a mechanical differential disposed in housing, saidmechanical differential positioned juxtapose respective transversechannels provided in lower and upper transaxle housing elements.